Eric,

The '251 also has a "source" mode which changes the instruction set to include 16 and 32 bit instructions while retaining 8051 std instructions. The machine includes an improved register file as well.

From a hardware standpoint the stack was made able to use external memory. The instruction cycle timing is reduced compare to std 8051. 1k of sram include on chip. The address space was increased to a logical 1 MB (256 practical) with a page mode to speed external memory access.

For ref:

http://developer.intel.com/design/mcs51/251.htm

regards,
p